1. Field of the Invention
The present invention relates to a radiant thermometer for detecting infrared rays radiated from an object to be measured to provide a temperature of the object based on the quantity of infrared rays radiated from a point in the object including a sighting or aiming system for indicating a particularly minute point to be measure.
2. Description of Related Art
A radiant thermometer is a passive instrument which detects infrared rays radiated from an object to be measured and is not brought into contact with the object to be measured. Accordingly, a radiant thermometer for measuring a temperature of a small portion of the object can be provided with a sighting system for indicating the position to be measured. Such a radiant thermometer is disclosed in FIG. 1 of Japanese Utility Model Application Laid-Open No. Sho 55-175835. In this radiant thermometer, infrared rays are condensated by means of a condensing lens to be incident upon a detector and a sighting light source and a sighting lens for focusing into an image of a sighting light on a point to be measured are provided on an optical axis in front of the condensing lens.
In such a radiant thermometer, the image of the sighting light source is focused on a point to be measured by means of the sighting lens, so that, even through the object to be measured or a portion to be measured is minute, any shift of a sighting position in the radial direction of the sighting lens can be comparatively easily removed by focusing the sighting light on the object to be measured.
However, the sighting light source and the sighting lens are provided on the optical axis on a side in front of the condensing lens, so that any infrared rays blocked by the sighting light source and the sighting lens cannot be incident upon the condensing lens and thus a problem has occurred in that infrared rays at these portions are lost.
Additionally, an opening angle or field angle from the point to be measured to the sighting lens is considerably smaller than that from the point to be measured to the condensing lens. Accordingly, a problem has occurred in that any shift in focus in the direction of the optical axis of the sighting lens can be difficult to identify and thus it is difficult to adjust the focus. Moreover, if a focus of the sighting light is shifted in the direction of the optical axis of the sighting lens, a difference in a diameter to be measured due to a dislocation on the point to be measured is considerably increased in comparison with the shift in focus, so that a problem has occurred also in that it is difficult to accurately position the light point to be measured within the minute portion to be measured.